Synthesis of Novel Bi-, Tri-, and Tetracyclic Nucleosides by Reaction

Synthesis of Novel Bi-, Tri-, and Tetracyclic Nucleosides by Reaction of a Common Cyclic Enamine Derived from TSAO-T with Nucleophiles Marı´a-Cruz Bonache,† Cristina Chamorro,†,§ Alessandra Cordeiro,† Marı´a-Jose´ Camarasa,† Marı´a-Luisa Jimeno,‡ and Ana San-Fe´lix*,† Instituto de Quı´mica Me´ dica, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain, and Centro de Quı´mica Orga´ nica “Manuel Lora Tamayo”, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain [email protected] Received July 27, 2004

We report here the efficient regio- and stereoselective synthesis of new polycyclic nucleosides using a common cyclic enamine (7) as the starting material. In fact, the reaction of 7, easily prepared by reaction of 5′-O-Tosyl TSAO-T under basic nonnucleophilic conditions (potassium carbonate), with different classes of nucleophiles, for example, nitrogen-, oxygen-, sulfur-, and carbon-based nucleophiles, or with amino acids afforded, with total regio- and stereoselectivity, new bi-, tri-, and tetracyclic nucleosides. This straighforward route represents an original and unambiguously regioand stereoselective pathway to these compounds. Some of these polycyclic nucleosides may be useful intermediates for a second series of reactions that may lead to the generation of structurally new nucleosides. Introduction The design and synthesis of bicyclic nucleosides has been of great importance in recent years. Because of the decrease in the conformational freedom that these compounds present, they have been used, as monomers, to synthesize antisense oligonucleotides with enhanced affinities for complementary RNA or DNA sequences1-7 and have been used in studies aimed to probe the * Fax: +34-91-5644853. † Instituto de Quı´mica Me ´ dica. ‡ Centro de Quı´mica Orga ´ nica “Manuel Lora Tamayo”. § Present address: Department of Medicinal Chemistry, Utrecht Institute for Pharmaceutical Sciences, Utrecht University, P.O. Box 80082, 3508 TB Utrecht, The Netherlands. (1) Herdewijn, P. Liebigs Ann. 1996, 1337. (2) (a) Kool, E. T. Chem. Rev. 1997, 97, 1473. (b) Herdewijn, P. Biochim. Biophys. Acta 1999, 1489. (c) Meldgaard, M.; Wengel, J. J. Chem. Soc., Perkin Trans. 1 2000, 3539. (3) (a) Tarko¨y, M.; Bolli, M.; Schweizer, B.; Leumann, C. Helv. Chim. Acta 1993, 76, 481. (b) Tarko¨y, M.; Leumann, C. Angew. Chem., Int. Ed. Engl. 1993, 32, 1432. (c) Egli, M.; Lubini, P.; Bolli, M.; Dobler, M.; Leumann, C. J. Am. Chem. Soc. 1993, 115, 5855. (d) Tarko¨y, M., Bolli, M.; Leumann, C. Helv. Chim. Acta 1994, 77, 716. (e) Litten, J. C.; Leumann, C. Helv. Chim. Acta 1996, 79, 1129. (f) Bolli, M.; Litten, J. C.; Schu¨tz, R.; Leumann, C. Chem. Biol. 1996, 3, 197. (g) Steffens, R.; Leumann, C. J. Am. Chem. Soc. 1997, 119, 11548. (4) (a) Altmann, K.-H.; Kesselring, R.; Francotte, E.; Rihs, G. Tetrahedron Lett. 1994, 35, 2331. (b) Altmann, K.-H.; Imwinkelried, R.; Kesselring, R.; Rihs, G. Tetrahedron Lett. 1994, 35, 7625. (c) Wang, G. Tetrahedron Lett. 2000, 41, 7139. (5) (a) Jones, R. J.; Swaminathan, S.; Milligan, J. F.; Wadwani, S.; Froehler, B. C.; Matteucci, M. D. J. Am. Chem. Soc. 1993, 115, 9816. (b) Wang, J.; Matteucci, M. D. Bioorg. Med. Chem. Lett. 1997, 7, 229. (c) Zou, R.; Matteucci, M. D. Tetrahedron Lett. 1996, 37, 941. (d) Van Aerschot, A.; Verheggen, I.; Hendrix, C.; Herdewijn, P. Angew. Chem., Int. Ed, Engl. 1995, 34, 1338. (6) (a) Singh, S. K.; Nielsen, P.; Koshkin, A. A.; Wengel, J. Chem. Commun. 1998, 455. (b) Koshkin, A. A.; Singh, S. K.; Nielsen, P.; Rajwanshi, V. K.; Kumar, R.; Meldgaard, M.; Olsen, C. E.; Wengel, J. Tetrahedron 1998, 54, 3607. (c) Singh, S. K.; Kumar, R.; Wengel, J. J. Org. Chem. 1998, 63, 10035.

conformational preferences of some receptors and enzymes involved in the metabolism or polymerization of nucleosides and nucleotides.8,9 On the other hand, a great diversity of bicyclic nucleosides have been synthesized for potential antiviral activity.10-15 Consequently, the design and synthesis of bicyclonucleosides remains a (7) (a) Ravn, J.; Nielsen, P. J. Chem. Soc., Perkin Trans. 1 2001, 985. (b) Meldgaard, M.; Wengel, J. J. Chem. Soc., Perkin Trans. 1 2000, 3539. (c) Kvaerno, L.; Wightman, R. H.; Wengel, J. J. Org. Chem. 2001, 66, 5106. (d) Sorensen, M. H.; Nielsen, C.; Nielsen, P. J. Org. Chem. 2001, 66, 4878. (8) Marquez, V. E.; Ezzitouni, A.; Russ, P.; Siddiqui, M. A.; Ford, H., Jr.; Feldman, R. J.; Mitsuya, H.; George, C.; Barchi, J. J., Jr. J. Am. Chem. Soc. 1998, 120, 2780. (9) (a) Marquez, V. E.; Russ, P.; Alonso, R.; Siddiqui, M. A.; Herna´ndez, S.; George, C.; Nicklaus, M. C.; Dai, F.; Ford, H., Jr. Helv. Chim. Acta 1999, 82, 2119. (b) Jacobson, K. A.; Ji, X.-d.; Li, A.-H.; Melman, N.; Siddiqui, M. A.; Shin, K.-J.; Marquez, V. E.; Ravi, R. G. J. Med. Chem. 2000, 43, 2196. (c) Comin, M. J.; Rodriguez, J. B.; Russ, P.; Marquez, V. Tetrahedron 2003, 59, 295. (d) Choi, Y.; George, C.; Comin, M. J.; Barchi, J. J., Jr.; Kim, H. S.; Jacobson, K. A.; Balzarini, J.; Mitsuya, H.; Boyer, P. L.; Hughes, S. H.; Marquez, V. E. J. Med. Chem. 2003, 46, 3292. (10) (a) Lescop, C.; Huet, F. Tetrahedron 2000, 56, 2995. (b) Marquez, V. E.; Siddiqui, M. A.; Ezzitouni, A.; Russ, P.; Wang, J.; Wagner, R. W.; Matteucci, M. D. J. Med. Chem. 1996, 39, 3739. (11) (a) Olsen, A. G.; Rajwanshi, V. K.; Nielsen, C.; Wengel. J. J. Chem. Soc., Perkin Trans. 1 2000, 3610. (b) Kvaerno, L.; Nielsen, C.; Wightman, R. H. Chem. Commun. 2000, 2903. (c) Tronchet, J. M. J.; Brenas, L.; Barbalat-Rey, F.; Zse´ly, M.; Geoffroy, M. Nucleosides Nucleotides 1998, 17, 1019. (12) (a) Okabe, M.; Sun, R.-C. Tetrahedron Lett. 1989, 30, 2203. (b) Beard, A. R.; Butler, P. I.; Mann, J.; Partlett, N. K. Carbohydr. Res. 1990, 205, 87. (13) Webb, T. R.; Mitsuya, H.; Broder, S. J. Med. Chem. 1988, 31, 1475. (14) Counde, O.-Y.; Kurz, W.; Eugui, E. M.; McRoberts, M. J.; Verheyden, J. P. H.; Kurz, L. J.; Walker, K. A. M. Tetrahedron Lett. 1992, 33, 41. (15) (a) Tronchet, J. M. J.; Zse´ly, M.; Lassout, O.; Barbalat-Rey, F.; Komaroni, I.; Geoffroy, M. J. Carbohydr. Chem. 1995, 14, 575. (b) Tronchet, J. M. J.; Zse´ly, M.; Ca´pek, K.; Komaroni, I.; De Clercq, E.; Balzarini, J. Nucleosides Nucleotides 1994, 13, 1871. 10.1021/jo048706p CCC: $27.50 © 2004 American Chemical Society

8758

J. Org. Chem. 2004, 69, 8758-8766

Published on Web 11/10/2004

Synthesis of Bi-, Tri-, and Tetracyclic Nucleosides SCHEME 1.

Reaction of 5′-O-Tosyl TSAO-m3T 1 with Alkylamines

challenging field of research with great potential from medicinal and synthetic points of view. Our research goal during the last few years has been the synthesis of hypermodified nucleosides as potential anti-HIV agents.16-18 In this context, we have developed a family of potent and specific inhibitors of HIV-1 reverse transcriptase.19,20 The prototype of this family is the thymine derivative TSAO-T (1; Figure 1).

signed a synthetic route that involved the nucleophilic attack of the 5′-O-Tosyl TSAO-m3T 1 derivative by different alkylamines. This reaction did not lead to the expected 5′-alkylamino derivative 2. Instead, new classes of highly functionalized bicyclic nucleosides 3 and 4,21 together with the 4′,5′-didehydro nucleoside 5, were obtained.21,22 Formation of the bicyclic nucleosides was explained by the attack of the alkylamine at the C-4′′ carbon atom of the 3-N-methyl thymine cyclic enamine 6 (Figure 2).21

FIGURE 1. Structure of TSAO-T (1).

As a part of our program to develop novel analogues of TSAO-T with an improved activity/toxicity profile, we were interested in the replacement of the 5′-TBDMS group by alkylamines (Scheme 1). For the synthesis of the 5′-alkylamino-substituted TSAO derivatives, we de(16) (a) Camarasa, M. J.; Dı´az-Ortiz, A.; Calvo-Mateo, A.; De las Heras, F. G.; Balzarini, J.; De Clercq, E. J. Med. Chem. 1989, 32, 1732. (b) Vela´zquez, S.; Camarasa, M. J. Tetrahedron 1992, 48, 1683. (17) Huss, S.; De las Heras, F. G. Tetrahedron 1991, 1727. (18) Vela´zquez, S.; Jimeno, M. L.; Huss, S.; Balzarini, J.; Camarasa, M. J. J. Org. Chem. 1994, 59, 7661. (19) (a) Camarasa, M. J.; Pe´rez-Pe´rez, M. J.; San-Fe´lix, A.; Balzarini, J.; De Clercq, E. J. Med. Chem. 1992, 35, 2721. (b) Pe´rez-Pe´rez, M. J.; San-Fe´lix, A.; Balzarini, J.; De Clercq, E.; Camarasa, M. J. J. Med. Chem. 1992, 35, 2988. (c) Vela´zquez, S.; San-Fe´lix, A.; Pe´rez-Pe´rez, M. J.; Balzarini, J.; De Clercq, E.; Camarasa, M. J. J. Med. Chem. 1993, 36, 3230. (20) (a) Balzarini, J.; Pe´rez-Pe´rez, M. J.; San-Fe´lix, A.; Schols, D.; Perno, C. F.; Vandamme, A. M.; Camarasa, M. J.; De Clercq, E. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 4392. (b) Balzarini, J.; Pe´rez-Pe´rez, M. J.; San-Fe´lix, A.; Vela´zquez, S.; Camarasa, M. J.; De Clercq, E. Antimicrob. Agents Chemother. 1992, 36, 1073. (c) Balzarini, J.; Pe´rezPe´rez, M. J.; San-Fe´lix, A.; Camarasa, M. J.; Bathurst, I. C.; Barr, P. J.; De Clercq, E. J. Biol. Chem. 1992, 267, 11831. (d) Camarasa, M. J.; San-Fe´lix, A.; Pe´rez-Pe´rez, M. J.; Vela´zquez, S.; Alvarez, R.; Chamorro, C.; Jimeno, M. L.; Pe´rez, C.; Gago, F.; De Clercq, E.; Balzarini, J. J. Carbohydr. Chem. 2000, 19, 451. (e) Camarasa, M. J.; San-Fe´lix, A.; Vela´zquez, S.; Pe´rez-Pe´rez, M. J.; Gago, F.; Balzarini, J. Curr. Top. Med. Chem. 2004, 4, 945.

FIGURE 2. Structure of cyclic enamines 6 and 7.

This was demonstrated by the isolation and subsequent treatment of 6 with different alkylamines to afford, in high yields, the new bicyclic nucleosides.21 Therefore, the cyclic enamine 6 could be considered as a versatile intermediate useful for obtaining bicyclic nucleosides in high yield. To consider this second synthetic strategy as a general and efficient approach for the synthesis of new bicyclic nucleosides, we decided to investigate the reaction of the nonmethylated thymine cyclic enamine 7 (Figure 2) with different classes of nucleophiles. Our results are described in this paper. Results Chemistry. The cyclic enamine 7 was prepared following the method previously reported for enamine 6.21 (21) Chamorro, C.; Luengo, S. M.; Bonache, M. C.; Vela´zquez, S.; Pe´rez-Pe´rez, M. J.; Camarasa, M. J.; Gago, F.; Jimeno, M. L.; SanFe´lix, A. J. Org. Chem. 2003, 68, 6695. (22) Chamorro, C.; Pe´rez-Pe´rez, M. J.; Rodriguez-Barrios, F.; Gago, F.; De Clercq, E.; Balzarini, J.; San-Fe´lix, A.; Camarasa, M. J. Antiviral Res. 2001, 50, 207.

J. Org. Chem, Vol. 69, No. 25, 2004 8759

Bonache et al.

Thus, treatment of 5′-O-Tosyl TSAO-T 823 under basic nonnucleophilic conditions (potassium carbonate), to avoid further reactions with the nucleophile, afforded the cyclic compound 7 in 70% yield (Scheme 2). SCHEME 2.

Synthesis of Cyclic Enamine 7

FIGURE 3. (A) gHMBC NMR correlations, indicated by arrows. (B) Relevant NOEs, indicated by arrows.

Formation of the cyclic enamine 7 could be explained by an intramolecular attack of the 4′′-amino group of the spiroaminooxathiole dioxide ring to the 5′-tosyl moiety21 (Scheme 2). With the cyclic enamine 7 in hand, our next objective focused on exploring its reactivity toward various nucleophiles, for example, oxygen-, sulfur-, carbon-, and nitrogen-based nucleophiles. Its reaction with amino acids was also investigated. First, we studied the reaction of 7 with (thio)alcohols. Thus, treatment of 7, under reflux with ethanol as the nucleophile, gave the tricyclic nucleoside 9 (70%), resulting from the nucleophilic attack of the alcohol to the C-4′′ carbon atom (Scheme 3). A similar reaction of 7 with SCHEME 3. Water

Reaction of 7 with Alcohols and

structure. In addition, a correlation was observed between the CH2 protons of the OCH2CH3 moiety and the C-4′′ carbon. The stereochemistry of the new stereogenic center created on C-4′′ was established as S on the basis of NOE difference experiments. Thus, irradiation of the signal at δ 1.26, corresponding to the methyl group of the OCH2CH3 moiety, caused the enhancement of the signals of the protons of the nucleobase H-6 and CH3-5 (Figure 3B), indicating that these protons and the ethanol group were at the same upper side of the furanose ring. Semiempirical calculations (using the Hyperchem package24) indicated that compound 9 (E ) -325.28 kcal mol-1) is more stable than its respective R isomer (E ) -299.24 kcal mol-1) by 26 kcal mol-1 in the gas phase. Reaction of 7 with ethanethiol afforded the tricyclic nucleoside 14 in 80% yield (Scheme 3). Next, we studied the reaction of 7 with water. Reaction of 7 with a 1:1 mixture of acetonitrile and water in the presence of potassium carbonate afforded the bicyclic nucleoside 15 (68% yield), in which a γ-lactam ring was fused to the ribose moiety (Scheme 3). The experimental results shown above with alcohols, thioethanol, or water could be explained (Scheme 4) by SCHEME 4. Proposed Mechanism for the Reaction of 7 with Alcohols and Water

different primary alcohols such as propanol, pentanol, isobutyl alcohol, and neopentanol gave the corresponding tricyclic derivatives 10-13 in high yields (60-83%; Scheme 3). However, reaction with tert-butyl alcohol or phenol as the nucleophiles only gave the unalterated starting compound 7. The presence of an AB system with signals at δ 3.73 and 3.98 (Jgem ) 13.9 Hz) in the 1H NMR spectrum of 9 was crucial for the identification of this compound. In the gradient heteronuclear multiple-bond correlation (gHMBC) experiment (Figure 3A), a long-range correlation between the H-5′ protons (δ 3.18 and 3.44) and the C-4′′ carbon (δ 97.7) was observed as corresponds to a cyclized

the attack of the nucleophile at the C-4′′ carbon atom of the spiroaminooxathiole dioxide to give intermediate I; a proton transfer at the next step would give the tricyclic derivatives 9-14. With water, formation of intermediate II and subsequent ring opening may occur to give the

(23) Ingate, S.; Pe´rez-Pe´rez, M. J.; De Clercq, E.; Balzarini, J.; Camarasa, M. J. Antiviral Res. 1995, 27, 281.

(24) HyperChem TM. Windows Molecular Modelling System, Release 5.1; HyperCube: Waterloo, Ontario, Canada, 1997.

8760 J. Org. Chem., Vol. 69, No. 25, 2004

Synthesis of Bi-, Tri-, and Tetracyclic Nucleosides

FIGURE 4. gHMBC NMR correlations, indicated by arrows.

bicyclic nucleoside 15, in which a γ-lactam ring appeared fused to the original ribose moiety. Next, we decided to investigate the reaction between 7 and a more nucleophilic compound such as sodium ethoxide (Scheme 5). SCHEME 5.

Reaction of 7 with Sodium Ethoxide

In this reaction, carried out at room temperature, we found that a new product (16) completely unrelated (as determined by 1H NMR) to the bicyclic nucleosides previously mentioned (9-15) was formed in 72% yield. The 1H and 13C NMR data of this compound revealed the presence of an (E)-vinylsulfonate moiety, together with an OH attached at the C-3′ position. In the gHMBC experiment (Figure 4), the absence of long-range correlations between the OCH2CH3 protons and the nucleoside carbons, together with the correlations observed between the H-5′ (δ 3.72 and 3.90) and H-3′′ protons (δ 4.76) with the C-4′′ carbon (δ 163.3), allowed the unambiguous assignment of 16 as a cyclized structure. In this reaction, it seems that the sodium ethoxide attacks at the sulfur of the SO2 group instead of at the C-4′′ carbon. This behavior has been previously observed in the reaction of sodium methoxide with other spiro derivatives of carbohydrates.25 The stereochemistry of the double bond of the vinylsulfonate moiety was established as E on the basis of NOE difference experiments. To prepare more functionalized derivatives from a common starting compound, we decided to synthesize the tricyclic nucleoside 17 (Scheme 6). Thus, reaction of 7 with sodium cyanide afforded the tricyclic nucleoside 17 in 75% yield (Scheme 6). This compound could be considered a very versatile intermediate for the synthesis of substituted tricyclic nucleosides by transforming the cyano moiety into an amide, carboxylic ester, amine, or other functional group. Hydrolysis of the cyano group was effectively carried out,26 using phase transfer conditions, by treatment of (25) Pe´rez-Pe´rez, M. J.; Camarasa, M. J.; Dı´az-Ortı´z, A.; San-Fe´lix, A.; De las Heras, F. G. Carbohydr. Res. 1991, 216, 399. (26) (a) Nair, V.; Purdy, D. F. Tetrahedron 1991, 47, 365. (b) Cacchi, S.; Misiti, D.; LaTorre, F. Synthesis 1989, 243.

FIGURE 5. Relevant NOEs, indicated by arrows.

17 with aqueous sodium hydroxide and hydrogen peroxide to furnish the carboxamide 18 in 65% yield (Scheme 6). Alcoholysis of the cyano group, by reaction of 17 with a saturated solution of HCl in MeOH, afforded the methoxycarbonyl derivative 19 in 60% yield (Scheme 6). Under these reaction conditions, concomitant deprotection of the 2′-O-tert-butyldimethylsilyl group was observed. Assignment of the absolute configuration at the C-4′′ position of the cyano derivative 17 was indirectly determined as S on the basis of NOE experiments carried out on the amide 18 and methyl ester 19 tricyclic derivatives (Figure 5). Thus, irradiation of the signal at δ 7.46 (compound 18), corresponding to one of the protons of the CONH2 moiety, and the signal at δ 3.86 (compound 19), corresponding to the CH3 of the methyl ester moiety, causes an enhancement of the signal for the H-2′ proton, indicating that all of these protons are located on the same upper face of the molecule. The absolute configurations for 18 and 19 were assumed to be the same as those of the corresponding cyano derivative 17 because no epimerization was expected in the transformation of this group. Catalytic hydrogenation of the CN group of 17 to generate the corresponding amine derivative 20 (Scheme 7) was attempted in methanol using 10% Pd/C as a catalyst. Initially, the reaction was carried out at room temperature and at atmospheric hydrogen pressure. However, under these conditions, the starting compound 17 remained unchanged, even upon prolonged reaction times. When the reaction was performed at 40 °C, the expected amine derivative 20 was not obtained; instead, the tetracyclic compound 21 was formed in 56% yield (Scheme 7). Assignment of the structure of tetracyclic nucleoside 21 was not obvious; this was achieved by 1H and 13C NMR spectroscopy using mono- and bidimensional techniques (gHMBC and gradient heteronuclear single-quantum coherence). The formation of compound 21 could be explained as follows (Scheme 8). The first step would be the catalytic hydrogenation of the cyanide group to give the amine 20 that was not isolated. According to the literature,27 alcohols may serve as carbonyl precursors to carry out the reductive alkylation of an amine in the presence of a reducing agent. Thus, we assume that reductive alkylation of 20 by methanol used as the solvent may occur to give imine 22. Once compound 22 is formed, an intramolecular attack of the NH of the pyrrolidine ring to the carbon of the imino moiety would give compound 21. This extremely easy attack suggests that the NH of the pyrrolidine moiety has a pronounced nucleophilic character; this was proved when compound 7 was sub(27) Rylander, P. N. Hydrogenation Methods; Academic Press: London, 1985; pp 82-93.

J. Org. Chem, Vol. 69, No. 25, 2004 8761

Bonache et al. SCHEME 6.

Synthesis of Tricyclic Nucleosides 17-19

SCHEME 7. Synthesis of the Tetracyclic Nucleoside 21

jected to reactions with compounds that contain carbonyl carbons. Thus, reaction of 7 with dimethyl malonate afforded compound 23 (55% yield; Scheme 9), and reaction of 7 with 4-p-methoxyphenylisocyanate afforded the ureido derivative 24 with a 58% yield. SCHEME 8.

The presence of the carbonyl group directly attached to the NH in compounds 23 and 24 was unambiguously determined by the strong downfield shift (δ 2.03) of the signal corresponding to the H-3′′ proton with respect to the same signal in the starting compound 7. Next, we focused on the reaction of the cyclic enamine 7 with primary and secondary amines. When 7 was reacted with an excess of a primary amine (propyl- or isopropylamine; Scheme 10), the bicyclic nucleosides 25 (65% yield) and 26 (63% yield) were obtained. A similar treatment of 7 with a secondary amine such as dimethylamine (2 M in THF) afforded the bicyclic nucleoside 27 in 69% yield (Scheme 10). As was previously proposed,21 formation of these compounds could be explained by the attack of the amine to the C-4′′ carbon atom of the spiroaminooxathiole ring followed by ring opening. Finally, we investigated the reaction of 7 with the benzyl or methyl ester derivatives of the following amino acids: L-valine [H-(L)-Val-OBn‚HCl], L-alanine [H-(L)Ala-OBn‚HCl], and L-glycine [H-(L)-Gly-OMe‚HCl]. The reaction, carried out in the presence of triethylamine (TEA), afforded the bicyclic nucleosides 28-30 in good yields (60-63%; Scheme 11). The presence of the (E)-vinylsulfonamide moiety was unambiguously determined by 1H and 13C NMR data. The

Proposed Mechanism for the Synthesis of 21

8762 J. Org. Chem., Vol. 69, No. 25, 2004

Synthesis of Bi-, Tri-, and Tetracyclic Nucleosides SCHEME 9.

Synthesis of 23 and 24

not detected; instead, the R-amino acid attacked at the sulfur of the SO2 group. An explanation for such behavior might be that the attack of the amino group present in the R-amino acid to the C-4′′ position is hindered for steric reasons. Therefore, we reasoned that insertion of a methylene group between the amino and the R carbon of the amino acid would enable the attack of the amino acid to the C-4′′ position, and this was indeed the case. Thus, the reaction of 7 and a β-amino acid such as β-L-alanine afforded compound 31 exclusively, resulting from the attack of the amino acid to the C-4′′ position (Scheme 11). Compound 31 was similar to the compounds (2527; Scheme 10) obtained from the reaction of 7 with primary and secondary amines. Conclusions

stereochemistry of this moiety was established as E on the basis of NOE difference experiments. SCHEME 10.

Reaction of 7 with Amines

Contrary to the reaction with amines (Scheme 10), in this reaction the compounds resulting from the nucleophilic attack of the R-amino acid to the C-4′′ position were SCHEME 11.

Reaction of 7 with Amino Acids

The work presented here clearly shows the potential of the cyclic enamine 7 for the regio- and steroselective synthesis of new highly functionalized polycyclic nucleosides, with different molecular skeletons. These very stable compounds were very efficiently obtained (high yields and easy purifications) by a one-step reaction of cyclic enamine 7 with different nucleophiles. In addition, the highly functionalized bicyclic nucleosides obtained from 7 have different sites of potential reactivity that enable the possibility of performing a second, third, and so forth series of reactions that may lead to the synthesis of new highly functionalized nucleosides with medicinal or synthetic applications. The compounds described herein are currently being evaluated for antiviral activity. In addition, some molecular modeling is in progress in order to rationalize our results.

Experimental Section The names of some polycyclic nucleosides in this section are given according to the von Baeyer nomenclature. However, for easy comparison, the assignments of the signals of the NMR spectra follow standard carbohydrate/nucleoside numbering (i.e., the furanose skeleton numbered 1′-5′), with the thymine moiety having the highest priority. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine}-3′-spiro-5′′-(4′′-amino-1′′,2′′oxathiole-2′′,2′′-dioxide) (7). To a solution of 5′-O-Tosyl TSAO-T 823 (0.30 g, 0.50 mmol) in dry acetonitrile (5 mL) was added potassium carbonate (0.08 g, 0.58 mmol). The solution was refluxed for 6 h and evaporated to dryness. The residue was dissolved in ethyl acetate (20 mL) and washed with water (2 × 20 mL). The organic layer was dried (Na2SO4), filtered, and evaporated to dryness. The residue was purified by CCTLC on the chromatotron [hexane/ethyl acetate (1:2)] to give 0.07 g (70%) of 7 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.87 (s, 3H), 3.92 (dd, 1H, J5′a,5′b ) 10.8 Hz), 4.06 (m, 1H), 4.94 (dd, 1H, J4′,5′a ) 5.7 Hz, J4′,5′a ) 8.9 Hz), 5.14 (d, 1H), 5.64 (s, 1H), 5.98 (d, 1H, J1′,2′ ) 8.9 Hz), 6.65 (bs, 1H), 7.69 (s, 1H), 10.51 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.1, 54.4, 74.1, 80.9, 88.6, 93.7, 111.9, 138.2, 157.2, 163.8, 168.8. MS (ES+) m/z: 458.1 (M + H)+. Anal. Calcd for C18H27N3O7SSi: C, 47.25; H, 5.95; N, 9.18. Found: C, 47.55; H, 5.65; N, 9.58. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine}-3′-spiro-5′′-(4′′-amino-4′′Sethoxy-1′′,2′′-oxathiolane-2′′,2′′-dioxide) (9). A solution of 7 (0.2 g, 0.46 mmol) in ethanol (6 mL) was heated in a sealed tube at 80 °C for 16 h. After evaporation of the solvent, the residue was purified by CCTLC on the chromatotron [hexane/

J. Org. Chem, Vol. 69, No. 25, 2004 8763

Bonache et al. ethyl acetate (1:2)] to give 0.15 g (70%) of 9 as a white foam. 1 H NMR [300 MHz, (CD3)2CO] δ: 1.26 (t, 3H, J ) 7.1 Hz), 1.85 (s, 3H), 3.18 (m, 1H), 3.44 (m, 1H), 3.57 (m, 1H), 3.73 (d, 1H, J ) 13.9 Hz), 3.83 (t, 1H), 3.90 (m, 1H), 3.98 (d, 1H), 4.62 (dd, 1H, J4′,5′a ) 4.3 Hz, J4′,5′b ) 6.1 Hz), 4.94 (d, 1H), 6.02 (d, 1H, J1′,2′ ) 6.8 Hz), 7.48 (s, 1H), 10.15 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.4, 15.6, 49.4, 54.7, 60.9, 72.1, 85.4, 92.2, 97.7, 99.5, 111.8, 137.5, 152.3, 166.2. MS (ES+) m/z: 526.2 (M + Na)+. Anal. Calcd for C20H33N3O8SSi: C, 47.70; H, 6.60; N, 8.34. Found: C, 47.61; H, 6.55; N, 8.69. General Procedure for the Synthesis of Bicyclic Nucleosides 10-14. To a solution of 7 (0.2 g, 0.46 mmol) in dry acetonitrile (10 mL) was added the corresponding (thio)alcohol (2.15 mmol). The reaction was heated at 80 °C for 18-24 h. After evaporation of the solvent, the residue was purified by CCTLC on the chromatotron [hexane/ethyl acetate (1:2)]. The analytical and spectroscopic data of the isolated products are indicated below for each reaction. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine}-3′-spiro-5′′-(4′′-amino-4′′Spropyloxy-1′′,2′′-oxathiolane-2′′,2′′-dioxide) (10). According to the general procedure, 7 was treated with propanol (0.4 mL, 2.15 mmol) to give 0.13 g (60%) of 10 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 0.98 (t, 3H, J ) 7.6 Hz), 1.66 (m, 2H), 1.83 (s, 3H), 3.18 (m, 1H), 3.44 (m, 1H), 3.46 (m, 1H), 3.60 (d, 1H, J ) 13.9 Hz), 3.80 (m, 3H), 3.97 (d, 1H), 4.63 (dd, 1H, J4′,5′a ) 4.2 Hz, J4′,5′b ) 5.9 Hz), 4.93 (d, 1H), 6.01 (d, 1H, J1′,2′ ) 6.5 Hz), 7.41 (s, 1H), 10.21 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 10.1, 11.3, 22.7, 50.2, 56.1, 65.5, 71.2, 84.5, 91.0, 92.7, 99.7, 112.4, 135.1, 151.3, 165.3. MS (ES+) m/z: 540.2 (M + Na)+. Anal. Calcd for C21H35N3O8SSi: C, 48.72; H, 6.81; N, 8.12. Found: C, 48.85; H, 6.95; N, 8.05. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine}-3′-spiro-5′′-(4′′-amino-4′′Spentyloxy-1′′,2′′-oxathiolane-2′′,2′′-dioxide) (11). Following the general procedure, 7 was treated with pentanol (0.2 mL, 2.15 mmol) to give 0.18 g (79%) of 11 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 0.90 (t, 3H, J ) 6.9 Hz), 1.39, 1.66 (m, 6H), 1.94 (s, 3H), 3.16 (m, 1H), 3.50 (m, 2H), 3.77 (t, 1H), 3.85 (m, 1H), 4.62 (dd, 1H, J4′,5′a ) 4.1 Hz, J4′,5′b ) 5.9 Hz), 4.92 (d, 1H), 6.01 (d, 1H, J1′,2′ ) 6.5 Hz), 7.49 (s, 1H), 10.22 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.5, 14.3, 23.1, 29.8, 49.7, 54.9, 64.7, 71.1, 84.9, 91.7, 98.7, 99.3, 111.1, 136.1, 151.3, 163.9. MS (ES+) m/z: 568.2 (M + Na)+. Anal. Calcd for C23H39N3O8SSi: C, 50.62; H, 7.20; N, 7.70. Found: C, 50.25; H, 6.85; N, 7.35. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine}-3′-spiro-5′′-(4′′-amino-4′′Sisobutyloxy-1′′,2′′-oxathiolane-2′′,2′′-dioxide) (12). According to the general procedure, 7 was treated with isobutyl alcohol (0.2 mL, 2.15 mmol) to give 12 (0.16 g, 72%) as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 0.98, 1.00 (d, 6H), 1.84 (s, 3H), 1.85 (m, 1H), 3.18 (dd, 1H), 3.19 (dd, 1H), 3.30 (dd, 1H), 3.54 (dd, 1H, J ) 6.5 Hz, J ) 8.9 Hz), 3.64 (d, 1H, J ) 13.7 Hz), 3.83 (t, 1H), 3.98 (d, 1H), 4.63 (dd, 1H, J4′,5′a ) 4.0 Hz, J4′,5′b ) 6.0 Hz), 4.95 (d, 1H), 6.01 (d, 1H, J1′,2′ ) 6.2 Hz), 7.35 (s, 1H), 10.12 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.6, 19.2, 19.3, 25.7, 48.9, 54.8, 70.9, 72.0, 85.0, 91.9, 98.3, 98.9, 111.3, 145.1, 150.9, 163.5. MS (ES+) m/z: 554.2 (M + Na)+. Anal. Calcd for C22H37N3O8SSi: C, 49.70; H, 7.01; N, 7.90. Found: C, 50.25; H, 6.85; N, 7.35. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine}-3′-spiro-5′′-(4′′-amino-4′′Sneopentyloxy-1′′,2′′-oxathiolane-2′′,2′′-dioxide) (13). Following the general procedure, 7 was treated with neopentanol (6 mL) to give 13 (0.2 g, 83%) as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.00 (s, 3H), 1.95 (s, 3H), 3.17 (m, 1H), 3.22 (d, 1H), 3.54 (m, 1H, J5′a,5′b ) 12.2 Hz), 3.57 (d, 1H, J ) 8.8 Hz), 3.84 (t, 1H), 4.62 (dd, 1H, J4′,5′a ) 4.1 Hz, J4′,5′b ) 6.2 Hz), 4.99 (d, 1H), 5.75 (s, 2H), 6.00 (d, 1H, J1′,2′ ) 6.3 Hz), 7.52 (s, 1H), 10.21 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.4, 29.2, 32.1, 49.0, 54.3, 65.5, 70.9, 84.9, 91.4, 98.1, 99.4, 111.5,

8764 J. Org. Chem., Vol. 69, No. 25, 2004

136.0, 151.4, 163.9. MS (ES+) m/z: 568.2 (M + Na)+. Anal. Calcd for C23H39N3O8SSi: C, 50.62; H, 7.20; N, 7.70. Found: C, 50.35; H, 6.75; N, 7.95. N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxy-βD-ribofuranosyl]thymine}-3′-spiro-5′′-(4′′-amino-4′′S-ethylthio-1′′,2′′-oxathiolane-2′′,2′′-dioxide) (14). According to the general procedure, 7 was treated with ethanethiol (0.160 mL) to give 0.178 g (80%) of 14 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.34 (t, 3H), 1.85 (s, 3H), 2.93 (q, 2H), 3.18 (m, 1H), 3.50 (m, 1H), 3.72 (t, 1H), 3.80 (d, 1H, J ) 13.9 Hz), 4.18 (d, 1H), 4.65 (dd, 1H, J4′,5′a ) 5.1 Hz, J4′,5′b ) 6.1 Hz), 4.82 (d, 1H), 6.02 (d, 1H, J1′,2′ ) 7.3 Hz), 7.55 (s, 1H), 10.41 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 13.1, 16.3, 19.2, 49.7, 54.9, 70.9, 92.1, 97.3, 99.1, 111.9, 132.9, 151.8, 163.2. MS (ES+) m/z: 542.3 (M + Na)+. Anal. Calcd for C20H33N3O7S2Si: C, 46.22; H, 6.40; N, 8.09. Found: C, 46.52; H, 6.30; N, 8.12. (1R,3R,4R,5R)-4-(tert-Butyldimethylsilyloxy)-5-mesyloxy-6-oxo-3-(thymin-1′-yl)-7-aza-2-oxabicyclo[3.3.0]octane (15). To a solution of 7 (0.05 g, 0.11 mmol) in a mixture of acetonitrile/water (1:1; 4 mL) was added potassium carbonate (0.02 g, 0.12 mmol). The reaction was refluxed for 10 h and evaporated to dryness. The residue was dissolved in ethyl acetate (20 mL) and washed with water (2 × 20 mL). The organic layer was dried (Na2SO4), filtered, and evaporated to dryness to give, after purification by CCTLC on the chromatotron [hexane/ethyl acetate (1:1)], 0.035 g (68%) of 15 as a white foam. 1H NMR [300 MHz, (CD3)2CO]) δ: 1.93 (s, 3H), 3.24 (s, 3H), 3.44 (m, 1H), 3.85 (dd, 1H, J4′,5′a ) 6.6 Hz, J5′a,5′b ) 11.0 Hz), 4.48 (d, 1H), 5.01 (d, 1H), 6.00 (d, 1H, J1′,2′ ) 7.1 Hz), 7.46 (bs, 1H), 7.69 (s, 1H), 10.20 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.7, 41.6, 46.7, 76.2, 81.6, 88.5, 91.0, 122.2, 136.6, 152.1, 164.4, 171.1. MS (ES+) m/z: 477.2 (M + H)+. Anal. Calcd for C18H29N3O8SSi: C, 45.46; H, 6.15; N, 8.84. Found: C, 45.69; H, 6.55; N, 8.69. (1R,3R,4R,5R)-4-(tert-Butyldimethylsilyloxy)-6-[1-(E)(ethoxysulfonyl)methylene]-5-hydroxy-3-(thymin-1′-yl)7-aza-2-oxabicyclo[3.3.0]octane (16). To a solution of 7 (0.1 g, 0.21 mmol) in dry acetonitrile (4 mL) was added sodium ethoxide (0.042 g, 0.63 mmol), and the resulting solution was stirred at room temperature for 5 min. The solution was treated with (5%) HCl until the pH was neutral and then concentrated to dryness. The residue was dissolved in ethyl acetate (20 mL) and washed with brine (2 × 20 mL). The organic layer was dried (Na2SO4), filtered, evaporated to dryness, and purified by CCTLC on the chromatotron [hexane/ ethyl acetate (1:2)] to give 0.075 g (72%) of 16 as a white foam. 1 H NMR [300 MHz, (CD3)2CO] δ: 1.27 (t, 3H, J ) 7.1 Hz), 1.84 (s, 3H), 3.72 (dd, 1H, J4′,5′a ) 2.7 Hz, J5′a,5′b ) 11.7 Hz), 3.90 (dd, 1H, J4′,5′b ) 6.2 Hz), 4.02 (q, 2H), 4.58 (d, 1H), 4.60 (dd, 1H), 4.76 (s, 1H), 4.91 (s, 1H), 6.01 (d, 1H, J1′,2′ ) 6.1 Hz), 7.03 (bs, 1H), 7.54 (s, 1H), 10.14 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.9, 13.4, 47.8, 54.7, 75.9, 80.8, 81.7, 83.0, 88.2, 109.1, 134.8, 152.5, 163.3, 164.4. MS (ES+) m/z: 504.2 (M + H)+. Anal. Calcd for C20H33N3O8SSi: C, 47.70; H, 6.60; N, 8.34. Found: C, 47.79; H, 6.75; N, 8.69. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine}-3′-spiro-5′′-(4′′-amino-4′′S-cyano-1′′,2′′-oxathiolane-2′′,2′′-dioxide) (17). A mixture of 7 (0.20 g, 0.43 mmol), sodium cyanide (0.07 g, 0.86 mmol), sodium bicarbonate (0.022 g, 0.43 mmol), ethyl acetate (4 mL), and water (2 mL) was stirred vigorously at room temperature for 10 h. The organic phase was separated, and the aqueous phase was washed with ethyl acetate (2 × 15 mL). The combined organic phases were dried over anhydrous sodium sulfate, filtered, and evaporated to dryness. The residue was chromatographed by CCTLC on the chromatotron [dichloromethane/methanol (30:1)] to give 0.156 g (75%) of 17 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.86 (s, 3H), 3.35 (m, 1H), 3.58 (m, 1H), 3.94 (d, 1H, J ) 13.9 Hz), 4.18 (t, 1H), 4.25 (d, 1H, H-3′′b), 4.71 (dd, 1H, H-4′, J4′,5′a ) 5.0 Hz, J4′,5′b ) 6.2 Hz), 4.85 (d, 1H, H-2′), 5.90 (d, 1H, H-1′, J1′,2′ ) 6.1 Hz), 7.63 (s, 1H, H-6), 10.26 (bs, 1H, NH-3). 13C NMR [75 MHz,

Synthesis of Bi-, Tri-, and Tetracyclic Nucleosides (CD3)2CO] δ: 12.0, 49.3, 52.7, 67.3, 76.1, 85.6, 90.6, 96.5, 111.4, 117.7, 135.7, 158.2, 162.3. MS (ES+) m/z: 485.6 (M + H)+. Anal. Calcd for C19H28N4O7SSi: C, 47.09; H, 5.82; N, 11.56. Found: C, 47.49; H, 5.75; N, 11.45. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine]-3′-spiro-5′′-(4′′-amino-4′′Scarbamoyl-1′′,2′′-oxathiolane-2′′,2′′-dioxide) (18). To a solution of 17 (0.150 g, 0.31 mmol) in dichloromethane was added 30% hydrogen peroxide (0.147 mL, 1.24 mmol), tetrabutylammonium hydrogen sulfate (0.021 g, 0.06 mmol), and 0.5 M aqueous sodium hydroxide (0.750 mL, 0.34 mmol). The reaction mixture was stirred at room temperature for 2 h. Water (20 mL) and dichloromethane (10 mL) were then added. The organic layer was separated, washed with brine (10 mL), and dried (Na2SO4). The solvent was removed under reduced pressure, and the residue was purified by CCTLC on the chromatotron [dichloromethane/methanol (20:1)] to give 0.11 g (65%) of 18 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.85 (s, 3H), 3.30 (d, 1H, J5′a,5′b ) 11.7 Hz), 3.45 (d, 1H), 3.67 (t, 1H), 3.68 (d, 1H, J ) 13.9 Hz), 4.35 (d, 1H), 4.64 (dd, 1H, J4′,5′a ) 4.6 Hz), 4.65 (d, 1H), 5.88 (d, 1H, J1′,2′ ) 6.6 Hz), 7.42 (s, 1H), 7.46 (bs, 1H), 8.20 (bs, 1H), 10.26 (bs, 1H). 13 C NMR [75 MHz, (CD3)2CO] δ: 12.0, 49.0, 54.6, 75.2, 77.3, 85.1, 89.1, 95.7, 112.0, 135.4, 151.2, 163.7, 172.3. MS (ES+) m/z: 503.1 (M + H)+. Anal. Calcd for C19H30N4O8SSi: C, 45.40; H, 6.02; N, 11.15. Found: C, 45.56; H, 5.45; N, 11.69. 5′,N4′′-Cyclo[1-(5′-deoxy-β-D-ribofuranosyl)thymine]-3′spiro-5′′-(4′′-amino-4′′S-methoxycarbonyl-1′′,2′′-oxathiolane-2′′,2′′-dioxide) (19). Compound 17 (0.1 g, 0.20 mmol) was treated with a saturated solution of HCl in MeOH (2 mL) at room temperature for 5 h and then evaporated to dryness. The residue was purified by CCTLC on the chromatotron [dichloromethane/methanol (10:1)] to give 0.05 g (60%) of 19 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.83 (s, 3H), 3.30 (d, 1H), 3.40 (d, 1H), 3.43 (t, 1H), 3.63 (d, 1H, J ) 14.6 Hz), 3.86 (s, 3H), 4.16 (d, 1H), 4.22 (d, 1H), 4.74 (dd, 1H, J4′,5′a ) 2.2 Hz, J4′,5′b ) 5.1 Hz), 5.23 (d, 1H, J ) 7.6 Hz), 5.98 (d, 1H, J1′,2′ ) 7.8 Hz), 7.45 (s, 1H), 10.11 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.4, 49.9, 54.1, 54.9, 74.3, 79.9, 86.3, 88.7, 99.0, 112.0, 137.2, 151.2, 163.2, 170.1. MS (ES+) m/z: 404.0 (M + H)+. Anal. Calcd for C14H17N3O9S: C, 41.69; H, 4.25; N, 10.42. Found: C, 41.75; H, 4.55; N, 10.59. (1R,5S,11R,13R,14R)-14-(tert-Butyldimethylsilyloxy)3,3-dioxo-13-(thymin-1′-yl)-7,9-diaza-2,12-dioxa-3thiatetracyclo[9.3.31,5.0.05,9]tetradecane (21). A solution of 17 (0.1 g, 0.20 mmol) in methanol (10 mL) containing Pd/C (10%; 0.020 g) was hydrogenated at 30 psi and 40 °C for 6 h. The reaction mixture was filtered, and the filtrate was evaporated to dryness, under reduced pressure. The residue was purified by CCTLC on the chromatotron [dichloromethane/ methanol (10:1)] to give 0.06 g (56%) of 21 as a white amorphous solid. 1H NMR [300 MHz, (CD3)2CO] δ: 1.83 (s, 3H), 3.05 (dd, 1H, J5′a,5′b ) 12.4 Hz), 3.36 (d, 1H, J ) 11.2 Hz), 3.47 (d, 1H, J ) 13.8 Hz), 3.62 (d, 1H), 3.67 (d, 1H), 3.69 (d, 1H), 3.83 (d, 1H, J ) 7.5 Hz), 4.02 (d, 1H), 4.70 (dd, 1H, J4′,5′a ) 4.0 Hz, J4′,5′b ) 5.2 Hz), 4.87 (d, 1H), 5.95 (d, 1H, J1′,2′ ) 7.5 Hz), 7.97 (s, 1H), 10.19 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.3, 54.7, 55.2, 56.1, 72.0, 72.5, 79.4, 87.4, 90.6, 97.5, 112.1, 137.1, 151.1 163.3. MS (ES+) m/z: 501.1 (M + H)+. Anal. Calcd for C20H32N4O7SSi: C, 47.98; H, 6.44; N, 11.19. Found: C, 47.75; H, 6.65; N, 11.60. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine}-3′-spiro-5′′-[4′′-(methoxycarbonylmethylcarbonyl)amino-1′′,2′′-oxathiole-2′′,2′′-dioxide] (23). A suspension of dimethyl malonate (0.035 mL, 0.31 mmol) and potassium carbonate (0.043 g, 0.31 mmol) in THF was stirred at room temperature for 30 min; then 7 (0.1 g, 0.21 mmol) and tetrabutylammonium bromide (0.069 g, 0.21 mmol) were added. The reaction mixture was refluxed for 6 h and then concentrated to dryness. The residue was dissolved in ethyl acetate (20 mL), washed with brine (2 × 20 mL), dried (Na2SO4), filtered, and evaporated to dryness. The residue thus

obtained was purified by CCTLC on the chromatotron [hexane/ ethyl acetate (1:1)] to give 0.064 g (55%) of 23 as a white amorphous solid. 1H NMR (300 MHz, DMSO-d6) δ: 1.84 (s, 3H), 2.84 (s, 2H), 3.72 (s, 3H), 4.21 (dd, 1H, J5′a,5′b ) 11.0 Hz), 4.43 (t, 1H), 4.97 (dd, 1H, J4′,5′a ) 5.8 Hz, J4′,5′b ) 8.8 Hz), 5.03 (d, 1H), 5.94 (d, 1H, J1′,2′ ) 9.0 Hz), 7.15 (s, 1H), 7.58 (s, 1H), 10.26 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.1, 35.5, 49.7, 54.5, 74.3, 80.8, 89.1, 93.9, 94.1, 112.5, 138.6, 157.2, 163.8, 165.5, 168.8, 171.0. MS (ES+) m/z: 558.1 (M + H)+. Anal. Calcd for C22H31N3O10SSi: C, 47.38; H, 5.60; N, 7.54. Found: C, 47.69; H, 5.55; N, 7.85. 5′,N4′′-Cyclo{1-[2′-O-(tert-butyldimethylsilyl)-5′-deoxyβ-D-ribofuranosyl]thymine}-3′-spiro-5′′-[4′′-(4-p-methoxyphenylcarbamoyl)amino-1′′,2′′-oxathiole-2′′,2′′-dioxide] (24). To a solution of 7 (0.1 g, 0.21 mmol) in acetonitrile was added 4-p-methoxyphenylisocyanate (0.062 mL, 0.63 mmol). The reaction was refluxed overnight and evaporated to dryness. The residue was dissolved in ethyl acetate (20 mL) and washed with water (2 × 20 mL). The organic layer was dried (Na2SO4), filtered, and evaporated to dryness. The residue was purified two successive times by CCTLC on the chromatotron, first using hexane/ethyl acetate (1:2) as eluent and then using dichloromethane/acetone (60:1), to give 0.073 g (58%) of 24 as a white foam. 1H NMR (300 MHz, DMSO-d6) δ: 1.84 (s, 3H), 2.84 (s, 2H), 3.72 (s, 3H), 4.21 (dd, 1H, J5′a,5′b ) 11.0 Hz), 3.45 (t, 1H), 4.97 (dd, 1H, J4′,5′a ) 5.8 Hz, J4′,5′b ) 8.8 Hz), 5.03 (d, 1H), 5.94 (d, 1H, J1′,2′ ) 9.0 Hz), 7.15 (s, 1H), 7.58 (s, 1H), 10.26 (bs, 1H). 13 C NMR [75 MHz, DMSO-d6] δ: 12.6, 54.6, 56.0, 74.5, 81.0, 89.6, 93.1, 94.1, 112.5, 114.2, 121.2, 130.0, 138.6, 152.3, 157.0, 157.3, 160.8, 163.9. MS (ES+) m/z: 607.3 (M + H)+. Anal. Calcd for C26H34N4O9SSi: C, 51.47; H, 5.65; N, 9.23. Found: C, 51.85; H, 5.95; N, 9.05. General Procedure for the Synthesis of Bicyclic Nucleosides 25-27. To a solution of 7 (0.1 g, 0.21 mmol) in dry acetonitrile (5 mL) was added an excess of the appropriate amine (0.84 mmol), and the reaction was heated in a sealed tube at 70 °C for 20 h. After evaporation of the solvent, the residue was purified by CCTLC on the chromatotron. The chromatography eluents and yields of the isolated products are indicated below for each reaction. (1R,3R,4R,5R)-4-(tert-Butyldimethylsilyloxy)-5-mesyloxy-6-propylamino-3-(thymin-1′-yl)-7-aza-2-oxabicyclo[3.3.0]oct-6-ene (25). The general procedure was followed using propylamine (0.07 mL, 0.87 mmol). Chromatography with dichlromethane/methanol (50:1) afforded 0.07 g (65%) of 25 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 0.75 (t, 3H), 1.45 (m, 2H), 1.86 (s, 3H), 3.10 (s, 2H), 3.19 (s, 6H), 3.71 (d, 1H, J5′a,5′b ) 15.5 Hz), 4.03 (dd, 1H), 4.71 (m, 2H), 5.23 (d, 1H, J4′,5′ ) 4.3 Hz), 5.77 (d, 1H, J1′,2′ ) 4.4 Hz), 7.21 (s, 1H), 10.21 (s, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 11.0, 12.9, 25.1, 40.6, 46.5, 61.3, 78.9, 85.9, 92.7, 96.8, 110.7, 137.3, 151.2, 160.4, 164.3. MS (ES+) m/z: 533.3 (M + H)+. Anal. Calcd for C22H40N4O7SSi: C, 49.60; H, 7.57; N, 10.52. Found: C, 49.30; H, 7.68; N, 10.35. (1R,3R,4R,5R)-4-(tert-Butyldimethylsilyloxy)-6-isopropylamino-5-mesyloxy-3-(thymin-1′-yl)-7-aza-2-oxabicyclo[3.3.0]oct-6-ene (26). The general procedure was followed using isopropylamine (0.06 mL, 0.84 mmol). Chromatography with dichloromethane/methanol (30:1) gave 0.071 g (63%) of 26 as a white foam. 1H NMR [300 MHz, (CD3) 2CO] δ: 1.00 (d, 3H, J ) 6.4 Hz), 1.12 (d, 3H), 1.86 (s, 3H), 3.26 (s, 3H), 3.46 (m, 1H), 3.79 (d, 1H, J5′a,5′b ) 15.8 Hz), 4.10 (dd, 1H), 4.74 (d, 1H), 5.13 (d, 1H, J4′,5′ ) 4.5 Hz), 5.75 (d, 1H, J1′,2′ ) 4.2 Hz), 7.29 (s, 1H), 10.23 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.9, 22.6, 23.1, 40.6, 45.6, 61.1, 78.4, 85.6, 92.6, 96.9, 110.7, 136.3, 151.9, 160.2, 164.5. MS (ES+) m/z: 533.2 (M + H)+. Anal. Calcd for C22H40N4O7SSi: C, 49.60; H, 7.57; N, 10.52. Found: C, 49.06; H, 7.55; N, 10.85. (1R,3R,4R,5R)-4-(tert-Butyldimethylsilyloxy)-6-(N,Ndimethylamino)-5-mesyloxy-3-(thymin-1′-yl)-7-aza-2oxabicyclo[3.3.0]oct-6-ene (27). The general procedure was

J. Org. Chem, Vol. 69, No. 25, 2004 8765

Bonache et al. followed using N,N-dimethylamine (0.06 mL, 0.84 mmol). Chromatography with dichloromethane/methanol (30:1) gave 0.08 g (69%) of 27 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.83 (s, 3H), 2.93 (s, 6H), 3.23 (s, 3H), 3.72 (d, 1H, J5′a,5′b ) 15.7 Hz), 4.04 (dd, 1H), 4.81 (d, 1H), 5.23 (d, 1H, J4′,5′ ) 4.5 Hz), 5.89 (d, 1H, J1′,2′ ) 4.3 Hz), 7.26 (s, 1H), 10.20 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.7, 40.9, 45.7, 61.2, 78.5, 85.4, 92.1, 96.3, 110.4, 136.1, 151.2, 160.5, 164.5. MS (ES+) m/z: 519.2 (M + H)+. Anal. Calcd for C21H38N4O7SSi: C, 48.63; H, 7.38; N, 10.80. Found: C, 48.41; H, 7.55; N, 10.45. General Procedure for the Synthesis of Nucleosides 28-31. To a solution of 7 (0.1 g, 0.21 mmol) in dry acetonitrile (4 mL) was added the corresponding C-protected amino acid (0.31 mmol) and triethylamine (0.047 mL, 0.31 mmol). The reaction was refluxed overnight and evaporated to dryness. The residue was dissolved in ethyl acetate (20 mL) and washed with water (2 × 20 mL). The organic layer was dried (Na2SO4), filtered, and evaporated to dryness. The residue was purified by CCTLC on the chromatotron with hexane/ethyl acetate (1:2). The yields of the isolated products together with the analytical and spectroscopic data are indicated below for each reaction. (1R,3R,4R,5R)-4-(tert-Butyldimethylsilyloxy)-5-hydroxy6-{1-(E)-{N-[1S-(benzyloxycarbonyl)isobutyl]aminosulfonyl}methylene}-3-(thymin-1′-yl)-7-aza-2oxabicyclo[3.3.0]octane (28). Following the general procedure, a solution of 7 and H-(L)-Val-OBn‚HCl (0.06 g, 0.31 mmol) was reacted to give 0.07 g (63%) of 28 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.02 (d, 3H, J ) 6.8 Hz), 1.05 (d, 3H), 1.89 (s, 3H), 3.60 (t, 2H, J4′,5′a ) 6.1 Hz, J5′a,5′b ) 12.2 Hz), 3.92 (t, 1H, J ) 6.1 Hz), 4.20 (t, 1H), 5.15 (d, 1H), 5.31 (s, 1H), 5.63 (d, 1H, J1′,2′ ) 6.8 Hz), 5.70 (s, 2H), 7.18 (t, 1H), 7.44 (m, 5H), 7.68 (s, 1H), 7.80 (d, 1H), 10.55 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.1, 16.9, 26.6, 48.5, 60.2, 72.1, 76.2, 84.0, 84.3, 91.2, 92.1, 111.4, 127.3, 128.7, 136.9, 140.3, 152.9, 163.5, 164.5, 172.4. MS (ES+) m/z: 665.8 (M + H)+. Anal. Calcd for C30H44N4O9SSi: C, 54.20; H, 6.67; N, 8.43. Found: C, 54.60; H, 6.12; N, 8.12. (1R,3R,4R,5R)-4-(tert-Butyldimethylsilyloxy)-5-hydroxy6-{1-(E)-{N-[1S-(benzyloxycarbonyl)ethyl]aminosulfonyl}methylene}-3-(thymin-1′-yl)-7-aza-2-oxabicyclo[3.3.0]octane (29). Following the general procedure, a solution of 7 and H-(L)-Ala-OBn‚HCl (0.06 g, 0.31 mmol) was reacted to give 0.08 g (63%) of 29 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.50 (t, 3H, J ) 7.2 Hz), 1.84 (s, 3H), 3.52 (d, 1H, J4′,5′a ) 3.0 Hz, J5′a,5′b ) 14.0 Hz), 3.63 (dd, 1H, J4′,5′b ) 6.1 Hz), 4.09 (m, 1H), 4.60 (s, 1H), 4.82 (d, 1H), 4.85 (dd, 2H, J ) 7.1 Hz, J ) 19.4 Hz), 5.81 (s, 1H), 6.12 (d, 1H, J1′,2′ ) 8.3 Hz), 7.03 (bs,

8766 J. Org. Chem., Vol. 69, No. 25, 2004

1H), 7.35 (m, 5H), 7.40 (s, 1H), 7.81 (d, 1H), 10.20 (bs, 1H). 13 C NMR [75 MHz, (CD3)2CO] δ: 12.1, 19.6, 50.9, 52.6, 68.3, 78.5, 84.0, 84.5, 91.1, 91.6, 111.4, 129.5, 135.4, 153.4, 158.2, 162.3. MS (ES+) m/z: 637.2 (M + H)+. Anal. Calcd for C28H40N4O9SSi: C, 52.81; H, 6.33; N, 8.80. Found: C, 52.20; H, 6.15; N, 8.25. (1R,3R,4R,5R)-4-(tert-Butyldimethylsilyloxy)-5-hydroxy6-{1-(E)-{N-[1S-(methoxycarbonyl)methyl]aminosulfonyl}methylene}-3-(thymin-1′-yl)-7-aza-2-oxabicyclo[3.3.0]octane (30). Following the general procedure, a solution of 7 and H-(L)-Gly-OMe‚HCl (0.04 g, 0.31 mmol) was reacted to give 0.07 g (60%) of 30 as a white foam. 1H NMR [300 MHz, (CD3)2CO] δ: 1.89 (s, 3H), 3.56 (d, 1H, J4′,5′a ) 3.1 Hz, J5′a,5′b ) 14.3 Hz), 3.65 (dd, 1H, J4′,5′b ) 6.3 Hz), 3.75 (s, 3H), 4.12 (m, 1H), 4.64 (s, 1H), 4.83 (d, 1H), 5.83 (s, 1H), 6.10 (d, 1H, J1′,2′ ) 8.1 Hz), 7.06 (bs, 1H), 7.45 (s, 1H), 7.80 (d, 1H), 10.15 (bs, 1H). MS (ES+) m/z: 547.2 (M + H)+. Anal. Calcd for C21H34N4O9SSi: C, 46.14; H, 6.27; N, 10.25. Found: C, 46.36; H, 6.85; N, 10.45. (1R,3R,4R,5R)-4-(tert-Butyldimethylsilyloxy)-5-mesyloxy-6-(2-methoxycarbonylethylamino)-3-(thymin-1′yl)-7-aza-2-oxabicyclo[3.3.0]oct-6-ene (31). Following the general procedure, a solution of 7 and H-β-Ala-OMe‚HCl (0.043 g, 0.31 mmol) was reacted to give 0.06 g (60%) of 31. 1H NMR (300 MHz, DMSO-d6) δ: 1.86 (s, 3H), 2.45 (t, 2H), 3.25 (s, 3H), 3.58 (s, 3H), 3.61 (m, 2H), 3.81 (dd, 1H), 3.99 (d, 1H, J5′a,5′b ) 15.5 Hz), 4.55 (d, 1H), 5.25 (d, 1H), 5.63 (d, 1H, J1′,2′ ) 7.6 Hz), 7.19 (s, 1H), 10.1 (bs, 1H). 13C NMR [75 MHz, (CD3)2CO] δ: 12.8, 35.3, 40.9, 45.9, 47.2, 61.5, 78.8, 85.4, 92.4, 96.5, 110.7, 136.5, 151.7, 163.2, 164.5, 172.0. MS (ES+) m/z: 577.2 (M + H)+. Anal. Calcd for C23H40N4O9SSi: C, 47.90; H, 6.99; N, 9.71. Found: C, 47.75; H, 6.55; N, 9.79.

Acknowledgment. We thank Susana Ruiz and Marı´a Jose´ Galeote for their excellent technical assistance. We thank the Ministry of Education of Spain for grants to C.C. and M.-C.B. The Spanish Ministerio de Ciencia y Tecnologı´a (Project SAF2003-07219-C0201) and the European Commission (Projects QLK2-CT2000-00291 and HPAW-CT-2002-90001) are acknowledged for their financial support. Supporting Information Available: General experimental methods, NMR procedures, and semiempirical calculations. This material is available free of charge via the Internet at http://pubs.acs.org. JO048706P